Why does Thomas Hobbes describe the state of nature as a state of war?
In the state of nature described by Hobbes, there are no laws and no rules. But human nature, being what it is, is pretty much the same in everyone. And as human nature, for Hobbes, is fundamentally selfish, everyone wants whatever they can get their hands on. Unfortunately, as there are no laws in Hobbes's state of nature, there is nothing to stop anyone from acquiring what they want except by prevailing force. And the only way for people to defend what is theirs from being taken by others is also by resorting to force. The state of nature, then, inevitably leads to conflict, a "war of all against all" as Hobbes calls it. Life is dictated not by notions of right or wrong, what's moral or immoral, but on the principle of "might is right," or "the law of the jungle."
But as human beings are endowed with rationality, they realize that things can't go on like this, otherwise there'll be no security of property, no arts, and no progress of any kind. That is why the people in a state of nature come together and invest a sovereign with absolute power to maintain peace and good order, providing the civil population with a modicum of stability in which to conduct their daily lives.
There are two characteristics of Hobbes's thought that lead him to equate the State of Nature with the State of War. The first is his embrace of empirical science, which leads him to conceptualize the human body as a machine.
By understanding the body as a machine, Hobbes conceptualizes humans as primarily concerned with keeping themselves alive (instead of primarily concerned with goodness or religious salvation). He also strips human beings and the State of Nature of their claim to inherent goodness. Since we are natural machines, we are only interested in avoiding pain and increasing pleasure.
For Hobbes, self-preservation of the body is the primary goal of all human beings and there is no pre-existing morality in the State of Nature. Morality as we usually understand it is something that humans invent as they come into societies. This means that even extreme actions, like murder, are to be expected in the State of Nature. Indeed, if everyone's most important goal is self-preservation, the State of Nature is virtually guaranteed to be a violent State of War.
This mindset allows Hobbes to reconsider the idea of natural law, which is the second characteristic of his thought that helps explain his scary State of Nature. Hobbes challenged the accepted wisdom of his time, which was influenced by the natural law doctrine of Saint Thomas Aquinas, by positing Laws of Nature that were unconcerned with good and evil.
Whereas Aquinas's first precept of natural law is "Good is to be done and pursued and evil avoided," Hobbes writes the following in Ch. XIV of Leviathan:
[The first law of nature is] 'that every man, ought to endeavour Peace, as farre as he can hope of obtaining it; and when he cannot obtain it, that he may seek, and use, all helps and advantages of Warre.' The first branch of which Rule, containeth the first, and Fundamental Law of Nature; which is, 'to seek Peace, and follow it.' The Second, the summe of the Right of Nature; which is, 'By all means we can, to defend our selves.'
For Hobbes, the state of nature simply is. In the State of Nature, human beings are all attempting to exercise their Right of Nature, which is the right to defend themselves. In the State of Nature, there is no God, there is no philosophy, there is no right and wrong. There is only the need for self-preservation.
Hobbes quickly and decisively shows his readers how such a State of Nature becomes the "war of all against all." A violent State of Nature/War hurts everyone's chances of self-preservation, so Hobbes concludes that all people will raitonally decide to seek peace (which is why "Seek peace" is the fundamental Law of Nature).
The only way to seek peace is to exit the state of nature (because it is always a state of war) and enter into a social contract. This is why most of Hobbes's Laws of Nature relate to contracts.